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Is it possible to have a layer only visible during designtime?
(using version 17.0.1)

Layer is used to group other objects but should not be visible during runtime.
Having it 'visible' during designtime, with for example a 1px border or distinguished background, makes it easyer to see it is there, select it for moving or altering it's size.

You can add a custom style to the layer. For example, visibility:hidden
Note however that this will also hide the layer's child elements.

I guess the term 'visible' is confusing here.

During disigntime I want to be aware that the layer exists (1px border or distinguished background)
while having the page published the layer should not be noticed.

Is this possible?

I'm sorry, there is no standard solution for that.